Lanthanum
Lanthanum is a chemical element; it has symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the first and the prototype of the lanthanide series, a group of 15 similar elements between lanthanum and lutetium in the periodic table. Lanthanum is traditionally counted among the rare-earth elements. Like most other rare-earth elements, its usual oxidation state is +3, although some compounds are known with an oxidation state of +2. Lanthanum has no biological role in humans but is used by some bacteria. It is not particularly toxic to humans but does show some antimicrobial activity.
Lanthanum usually occurs together with cerium and the other rare-earth elements. Lanthanum was first found by the Swedish chemist Carl Gustaf Mosander in 1839 as an impurity in cerium nitrate hence the name lanthanum, from the ancient Greek λανθάνειν, meaning 'to lie hidden'. Although it is classified as a rare-earth element, lanthanum is the 28th most abundant element in the Earth's crust, almost three times as abundant as lead. In minerals such as monazite and bastnäsite, lanthanum composes about a quarter of the lanthanide content. It is extracted from those minerals by a process of such complexity that pure lanthanum metal was not isolated until 1923.
Lanthanum compounds have numerous applications including catalysts, additives in glass, carbon arc lamps for studio lights and projectors, ignition elements in lighters and torches, electron cathodes, scintillators, and gas tungsten arc welding electrodes. Lanthanum carbonate is used as a phosphate binder to treat high levels of phosphate in the blood accompanied by kidney failure.
Characteristics
Physical
Lanthanum is the first element and prototype of the lanthanide series. In the periodic table, it appears to the right of the alkaline earth metal barium and to the left of the lanthanide cerium. Lanthanum is generally considered the first of the f-block elements by authors writing on the subject. The 57 electrons of a lanthanum atom are arranged in the configuration 5d6s, with three valence electrons outside the noble gas core. In chemical reactions, lanthanum almost always gives up these three valence electrons from the 5d and 6s subshells to form the +3 oxidation state, achieving the stable configuration of the preceding noble gas xenon. Some lanthanum compounds are also known, but they are usually much less stable. Lanthanum monoxide produces strong absorption bands in some stellar spectra.Among the lanthanides, lanthanum is exceptional as it has no 4f electrons as a single gas-phase atom. Thus it is only very weakly paramagnetic, unlike the strongly paramagnetic later lanthanides. However, the 4f shell of lanthanum can become partially occupied in chemical environments and participate in chemical bonding. For example, the melting points of the trivalent lanthanides are related to the extent of hybridisation of the 6s, 5d, and 4f electrons, and lanthanum has the second-lowest melting point among them: 920 °C. This chemical availability of f orbitals justifies lanthanum's placement in the f-block despite its anomalous ground-state configuration.
The lanthanides become harder as the series is traversed: as expected, lanthanum is a soft metal. Lanthanum has a relatively high resistivity of 615 nΩm at room temperature; in comparison, the value for the good conductor aluminium is only 26.50 nΩm. Lanthanum is the least volatile of the lanthanides. Like most of the lanthanides, lanthanum has a hexagonal crystal structure at room temperature. At 310 °C, lanthanum changes to a face-centered cubic structure, and at 865 °C, it changes to a body-centered cubic structure.
Chemical
As expected from periodic trends, lanthanum has the largest atomic radius of the lanthanides. Hence, it is the most reactive among them, tarnishing quite rapidly in air, turning completely dark after several hours and can readily burn to form lanthanum oxide,, which is almost as basic as calcium oxide. A centimeter-sized sample of lanthanum will corrode completely in a year as its oxide spalls off like iron rust, instead of forming a protective oxide coating like aluminium, scandium, yttrium, and lutetium. Lanthanum reacts with the halogens at room temperature to form the trihalides, and upon warming will form binary compounds with the nonmetals nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic. Lanthanum reacts slowly with water to form lanthanum hydroxide,. In dilute sulfuric acid, lanthanum readily forms the aquated tripositive ion : This is colorless in aqueous solution since has no d or f electrons. Lanthanum is the strongest and hardest base among the rare earth elements, which is again expected from its being the largest of them.Some lanthanum compounds are also known, but they are much less stable. Therefore, in officially naming compounds of lanthanum its oxidation number always is to be mentioned.
Isotopes
Naturally occurring lanthanum is made up of two isotopes, the stable and the primordial long-lived radioisotope. is by far the most abundant, making up 99.911% of natural lanthanum: it is produced in the s-process and the r-process. It is the only stable isotope of lanthanum. The very rare isotope is one of the few primordial odd–odd nuclei, with a long half-life of It is one of the proton-rich p-nuclei which cannot be produced in the s- or r-processes., along with the even rarer tantalum-180m|, is produced in the ν-process, where neutrinos interact with stable nuclei. All other lanthanum isotopes are synthetic: with the exception of, which has a half-life of about 60,000 years, all of them have half-lives less than two days, and most have half-lives less than a minute. The isotopes and occur as fission products of uranium.Compounds
is a white solid that can be prepared by direct reaction of its constituent elements. Due to the large size of the ion, adopts a hexagonal 7-coordinate structure that changes to the 6-coordinate structure of scandium oxide and yttrium oxide at high temperature. When it reacts with water, lanthanum hydroxide is formed: a lot of heat is evolved in the reaction and a hissing sound is heard. Lanthanum hydroxide will react with atmospheric carbon dioxide to form the basic carbonate.Lanthanum fluoride is insoluble in water and can be used as a qualitative test for the presence of. The heavier halides are all very soluble deliquescent compounds. The anhydrous halides are produced by direct reaction of their elements, as heating the hydrates causes hydrolysis: for example, heating hydrated produces.
Lanthanum reacts exothermically with hydrogen to produce the dihydride, a black, pyrophoric, brittle, conducting compound with the calcium fluoride structure. This is a non-stoichiometric compound, and further absorption of hydrogen is possible, with a concomitant loss of electrical conductivity, until the more salt-like is reached. Like and, is probably an electride compound.
Due to the large ionic radius and great electropositivity of, there is not much covalent contribution to its bonding and hence it has a limited coordination chemistry, like yttrium and the other lanthanides. Lanthanum oxalate does not dissolve very much in alkali-metal oxalate solutions, and decomposes around 500 °C. Oxygen is the most common donor atom in lanthanum complexes, which are mostly ionic and often have high coordination numbers over is the most characteristic, forming square antiprismatic and dodecadeltahedral structures. These high-coordinate species, reaching up to coordination number 12 with the use of chelating ligands such as in, often have a low degree of symmetry because of stereo-chemical factors.
Lanthanum chemistry tends not to involve due to the electron configuration of the element: thus its organometallic chemistry is quite limited. The best characterized organolanthanum compounds are the cyclopentadienyl complex, which is produced by reacting anhydrous with in tetrahydrofuran, and its methyl-substituted derivatives.
History
In 1751, the Swedish mineralogist Axel Fredrik Cronstedt discovered a heavy mineral from the mine at Bastnäs, later named cerite. Thirty years later, the fifteen-year-old Wilhelm Hisinger, from the family owning the mine, sent a sample of it to Carl Scheele, who did not find any new elements within. In 1803, after Hisinger had become an ironmaster, he returned to the mineral with Jöns Jacob Berzelius and isolated a new oxide which they named ceria after the dwarf planet Ceres, which had been discovered two years earlier. Ceria was simultaneously independently isolated in Germany by Martin Heinrich Klaproth. Between 1839 and 1843, ceria was shown to be a mixture of oxides by the Swedish surgeon and chemist Carl Gustaf Mosander, who lived in the same house as Berzelius and studied under him: he separated out two other oxides which he named lanthana and didymia. He partially decomposed a sample of cerium nitrate by roasting it in air and then treating the resulting oxide with dilute nitric acid. That same year, Axel Erdmann, a student also at the Karolinska Institute, discovered lanthanum in a new mineral from Låven island located in a Norwegian fjord.Finally, Mosander explained his delay, saying that he had extracted a second element from cerium, and this he called didymium. Although he did not realise it, didymium too was a mixture, and in 1885 it was separated into praseodymium and neodymium.
Since lanthanum's properties differed only slightly from those of cerium, and occurred along with it in its salts, he named it from the Ancient Greek . Relatively pure lanthanum metal was first isolated in 1923.
Occurrence and production
Lanthanum makes up 39 mg/kg of the Earth's crust, behind neodymium at 41.5 mg/kg and cerium at 66.5 mg/kg. Despite being among the so-called "rare-earth metals", lanthanum is thus not rare at all, but it is historically named so because it is rarer than "common earths" such as lime and magnesia, and at the time it was recognized only a few deposits were known. Lanthanum is also considered a "rare-earth" metal because the process to mine it is difficult, time-consuming, and expensive. Lanthanum is rarely the dominant lanthanide found in the rare-earth minerals, and in their chemical formulas it is usually preceded by cerium. Rare examples of La-dominant minerals are monazite- and lanthanite-.The ion is similarly sized to the early lanthanides of the cerium group that immediately follow in the periodic table, and hence it tends to occur along with them in phosphate, silicate and carbonate minerals, such as monazite and bastnäsite, where M refers to all the rare-earth metals except scandium and the radioactive promethium. Bastnäsite is usually lacking in thorium and the heavy lanthanides, and the purification of the light lanthanides from it is less involved. The ore, after being crushed and ground, is first treated with hot concentrated sulfuric acid, evolving carbon dioxide, hydrogen fluoride, and silicon tetrafluoride: the product is then dried and leached with water, leaving the early lanthanide ions, including lanthanum, in solution.
The procedure for monazite, which usually contains all the rare earths as well as thorium, is more involved. Monazite, because of its magnetic properties, can be separated by repeated electromagnetic separation. After separation, it is treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with sodium hydroxide to pH 3–4. Thorium precipitates out of solution as hydroxide and is removed. After that, the solution is treated with ammonium oxalate to convert rare earths to their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in. Lanthanum is separated as a double salt with ammonium nitrate by crystallization. This salt is relatively less soluble than other rare-earth double salts and therefore stays in the residue. Care must be taken when handling some of the residues as they contain radium-228|, the daughter of, which is a strong gamma emitter. Lanthanum is relatively easy to extract, as it has only one neighbouring lanthanide cerium, which can be removed by making use of its ability to be oxidised to the +4 state; thereafter, lanthanum may be separated out by the historical method of fractional crystallization of, or by ion-exchange techniques when higher purity is desired.
Lanthanum metal is obtained from its oxide by heating it with ammonium chloride or fluoride and hydrofluoric acid at 300–400 °C to produce the chloride or fluoride:
This is followed by reduction with alkali or alkaline-earth metals in vacuum or argon atmosphere:
Also, pure lanthanum can be produced by electrolysis of molten mixture of anhydrous and or at elevated temperatures.